Recently, it has been practiced in construction sites to measure the position of a monitoring point set on a construction machine by employing a three-dimensional position measuring device, such as a GPS, for management of work. One typical example of the monitoring point is a position of an operating mechanism of the construction machine, for example, a bucket fore end position of a hydraulic excavator. By measuring the bucket fore end position, progress situations of work under execution can be confirmed by comparing measured data with preset landform data and target configuration data, and the management of the work can be performed during the execution. Even after the execution, the management of the work can also be performed by producing completed configuration data (e.g., excavated landform data) from the measured data.
As disclosed in JP,A 2001-98585, for example, a prior-art position measuring system of that type includes the steps of measuring a three-dimensional position of an excavation operating mechanism (bucket) to compute a working plane, determining an intersect line of the working plane with a three-dimensional target landform, displaying relative positions of the intersect line and the excavation operating mechanism, and measuring a fore end position of the bucket by using angle sensors for a boom, an arm and the bucket and GPS's installed in front and rear portions of a machine body.